Process for improving the bunkerability of coal

ABSTRACT

This invention relates to the treatment of a coal slurry and particularly to the adiabatic flashing of the coal slurry solids removed from a centrifuge to effect a reduction in the moisture content present before the coal is sent to the pulverized coal mill or cyclone burner to improve the bunkerability of the solids.

[mite States Patent [1 1 Wasp Feb. 18, 1975 [54] PROCESS FOR IMPROVINGTHE 3,313,251 4/1967 Jonakin 110/7 R BUNKERABILITY 0 COAL- 3,340,8309/1967 Frey et a1..... 110/7 R 3,359,040 12/1967 Every et a1 44/51 X[75] Inventor: Edward J. Wasp, San Rafael, Calif. [73] Assignee: BechtelInternational Corporation,

San Francisco, Calif. Primary Examiner-C. Dees [22] Filed: Mar. 14, 1973[21] Appl. No.:'341,132

Related U.S. Application Data 57 ABSTRACT [63] Continuation-impart ofSer. No, 248,026, April 27,

1972, abandoned.

This mventron relates to the treatment of a coal slurry 52 U.S. c1 44/1R and Particularly to the adiabatic flashing of the Coal 51 Int.c1019/00 Slurry Solids removed from a centrifuge effect a [58] Field ofSearch 044/] R 6 0/7 R duction in the moisture content present beforethe coal is sent to the pulverized coal mill or cyclone [56] ReferencesCited burner to improve the bunkerability of the solids.

UNITED STATES PATENTS 2,162,200 6/1939 Kiesskalt et a1 44/51 3 Claims, 2Drawing Figures PROCESS FOR IMPROVING THE BUNKERABILITY OF COAL Thisapplication is a continuation-in-part of my application Ser. No. 248,026flled Apr. 27, 1972, now abandoned.

BACKGROUND OF THE INVENTION Heretofore the usual method of handling acoal slurry removed from a centrifuge has been to feed the slurrydirectly into a mill to pulverize the coal after which it is burned.This concept is shown in US. Pat. No. 3,340,830.

SUMMARY OF THE INVENTION Total Inherent Surface Moisture MoistureMoisture Coal before flashing but after centrifuging at 190F. 32% 20%12% Coal after flashing 190 to 100F. 28% 20% 8% From the above table,the reduction in surface moisture is from 12% to 8%, or about one-third.Not only is 4%-less total mass sent through the boiler along with thefuel, but as an additional benefit, flashing permits easier handling ofthe coal. Coal with surface moisture of 8% is more handleable in abunker than is coal with 12% surface moisture. It is preferred tomaintain the surface moisture between about 5% and 8%. Thus, the coal isnot dusty as long as the surface moisture is greater than 5% and,contrarywise, coal bunkerability decreases when the surface moisturebecomes greater than about 8%.

The overall concept consists of the following sequential steps:

1. Heating the slurry to an elevated temperature, e.g.,

approximately 190F.

2. A mechanical dewatering operation.

3. Flashing of the heated cake from the dewatering device in a contactorwhich is swept by steam and a purge gas, more thanhalf of the heatrequired being supplied by the sensible heat in the coal.

4. Conveying the flashed cake to bunkers or other storage beforeutilization.

This invention is applicable to boiler fuel or a flocculated cokingcoal. The flocculation of coal by adding oils has been known for sometime. Fairly light and expensive oils have been used. The major drawbackof these oils has been that the agglomerates formed are not stable. Thefine coal so flocculated becomes dusty in less than a day. The oil isapparently absorbed in the coal and because it ceases to be on thesurface of the coal, it loses the ability to keep the finesagglomerated.

I prefer to use heavy oils preferably of a residual nature and which arequite viscous or even solid at room temperatures. As will appear, themixture is heated at a high enough temperature so that the heavy oilsare quite fluid to permit the flocculation to occur. I have '2 usedheavy hydrocarbons having a viscosity at 190F. of less than 3centipoises and a viscosity greater than 12 centipoises at F. Theresulting inverted mixture is centrifuged hot and then the resultingmixture tumbled in contact with a sweep gas, such as air, to remove theflashing vapors. The resulting product is not only agglomerated as astable agglomerate, but in addition,

the flashing results in a total moisture content such as 6% to 8% whichis generally only achievable by thermal drying. 1

A further advantage of the present invention is that by cooling the coalafter the centrifuge step, the coal is fed much more readily, enablingthe width of the bunker from which the coal is fed to be materiallyreduced as the temperature of the coal is reduced.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a flowsheet showing typical stepsin the practice of the invention.

FIG. 2 shows the variation of the bunker opening required against thetemperature of the coal in the bunker.

DESCRIPTION OF THE PREFERRED I EMBODIMENT Referring to FIG. 1, the coalslurry is delivered from a pipeline or cleaning plant through line 6 ata temperature of about 60F. The coal can have a tar-oil mixture applied.The slurry is then passed through a heat exchanger 7 wherein it isheated by the effluent from centrifuge 8, the effluent being deliveredthrough line 9 to the heat exchanger 7, usually at a temperature ofabout 190F. The effluent is thereafter sent through line 11 to aclarifier after which it may be discharged or reused.

From the heat exchanger, the coal slurry is passed through line 12 intoa preheater 113 where it is heated by steam from line 14 until itstemperature approximates 190F. The heated slurry is then sent throughline 16 to the centrifuge 8. The cake typically delivered by thecentrifuge through line 17 has a moisture content of approximately27%-31% at 190F. The centrifuge cake is then sent through a contactor 18where it is cooled by a purge gas delivered from line 19 to reduce thetemperature of the cake to say F. The steam and purge gas are removedthrough line 21 while the cooled centrifuge cake is'withdrawn throughline 22 and is sent to a bunker.

As will be seen from FIG. 2, there is significant decrease in the bunkerwidth opening as the temperature of the cake approximates F. Thisenables the coal to be fed more readily and more after the manner offeeding coal transported by rail rather than as the usual slurried coal.7

I claim:

1. A process for improving the bunkerability of coal which has beentransported as a coal slurry, comprising the steps of heating the slurryto a temperature of about F, mechanically dewatering the slurry toproduce a heated cake, adiabatically flashing the heated cake to reducethe temperature thereof to about 100F, contacting the cake with a purgegas and reducing the surface moisture of the cake to between about 5%and about 8%. I

2. A process as in claim 1 including the step of sweeping the cake withhot steam from an external source to aid the flashing operation, morethan half of the dryer load being due to the sensible heat in the cake.

3. A process as in claim 1 wherein the coal slurry is mixed with a heavyhydrocarbon before being heated, the hydrocarbon having a viscosity at190F. of less poises.

1. A PROCESS FOR IMPROVING THE BUNKERABILITY OF COAL WHICH HAS BEENTRANSPORTED AS A COAL SLURRY, COMPRISING THE STEPS OF HEATING THE SLURRYTO A TEMPERATURE OF ABOUT 190*F, MECHANICALLY DEWATERING THE SLURRY TOPRODUCE A HEATED CAKE, ADIABATICALLY FLASHING THE HEATED CAKE TO REDUCETHE TEMPERATURE THEREOF TO ABOUT 100*F, CONTACTING THE CAKE WITH A PURGEGAS AND REDUCING THE SURFACE MOISTURE OF THE CAKE TO BETWEEN ABOUT 5%AND ABOUT 8%.
 2. A process as in claim 1 including the step of sweepingthe cake with hot steam from an external source to aid the flashingoperation, more than half of the dryer load being due to the sensibleheat in the cake.
 3. A process as in claim 1 wherein the coal slurry ismixed with a heavy hydrocarbon before being heated, the hydrocarbonhaving a viscosity at 190*F. of less than 3 centipoises and at 60*F. ofgreater than 12 centipoises.